The present invention relates to a photodiode having a PIN construction, and more particularly to a PIN photodiode having a high speed of response to light and method of fabrication of the same.
One general PIN photodiode is described in "Hydro-genated amorphous silicon PIN diodes with high rectification ratio" by Kouich Seki, et al in Journal of Noncrystalline Solids, 59 & 60 (1983), pages 1179-1182, published by North-Holland Publishing Company. A general PIN photodiode will be described with reference to FIG. 1 of the accompanying drawings.
FIG. 1 shows in cross section of a general PIN photodiode. In FIG. 1, a transparent substrate 11 is composed of a transparent substrate portion 12 such as of glass and a transparent conductive layer 13 deposited on the transparent substrate portion 12 and serving as a lower electrode. A PIN diode 14 serving as a photoelectric transducer comprises successive photoconductive layer s 15 through 17 such as of amorphous silicon (a - Si), the layer 15 being composed of a p-type a - Si layer, the layer 16 of an i-type (intrinsic) a - Si layer, and the layer 17 of an n-type a - Si layer. An upper electrode 18 is disposed on the n-type a - Si layer 17 of the photoconductive transducer 14. The upper electrode 18 has an electrode region lying inwardly of the regions of the lower photoconductive layers 15 through 17 in plan.
Operation of the prior PIN photodiode shown in FIG. 1 will be described below. When light L is illuminated through the transparent substrate 11 on the photoconductive transducer 14, electron-and-hole pairs are generated in the PIN diode 14 dependent on the intensity of the light L and picked up as a current between the upper electrode 18 and the lower electrode 13. The PIN diode 14 is reverse biased, and substantially no current will flow between the electrodes when no light is illuminated on the PIN diode 14. Therefore, the illumination of the light L can be detected by the current flowing between the electrodes 13, 18.
As illustrated in FIG. 1, the peripheral edges of the region of the upper electrode 18 are retracted a distance l inwardly from the peripheral edges of the photoconductive layers of the PIN diode 14. This is because if the electrode 18 and the photoconductive layers 15 through 17 were of the same size, a short circuit would occur between the lower electrode 13 and the upper electrode material which might be brought into contact with the electrode 13 due to a mask displacement when the upper electrode 18 is patterned after the photoconductive layers 15 through 17 have been patterned. Even if no mask displacement were caused, a leakage current between the electrodes 13, 18 would be increased through defects in the vicinity of the end faces of the photoconductive layers if the regions of the upper electrode 18 and photoconductive layers 15 through 17 were of the same size in plan, since the total thickness of the photoconductive layers 15 through 17 is about 1 .mu.m at maximum. Such an increased leakage current would impair the characteristics of the PIN diode as an optical sensor device.
For the reasons described above, the peripheral edges of the PIN diode 14 is spaced the distance l from the peripheral edges of the electrode 18. Since a region of the PIN diode 14 within the distance l from the peripheral edges is not in contact with the upper electrode 18, such a non-contacting region is not subjected to the biasing voltage between the electrodes 13, 18. Thus, no biasing voltage is impressed on electrons and holes generated in the photoconductive layers within the distance l. It takes a long time for these electrons and holes to reach the electrodes 13, 18, and therefore the speed of response (response to light) of the current between the electrodes with respect to ON and OFF states of the incident light L is low.